One of the sources of pollutants to the air comprises the exhaust gases from automobiles and non-mobile combustion sources. Such exhaust gases contain not only carbon monoxide and hydrocarbons, but also various nitrogen oxide gases. The concentrations of such contaminants in the air will have to be lowered in the near future. It is contemplated that relatively low levels of nitrogen oxide gases will be established for automobile exhaust gases. It is also contemplated that substantially all of the carbon monoxide and unburned hydrocarbons that are present in an exhaust gas will have to be eliminated.
Various systems for treating such exhaust gas contaminants have been proposed. Such proposed systems include a two-stage catalytic converter. The first stage in this catalytic converter treats the exhaust gases that are coming directly from the exhaust system under reducing conditions. In the first stage, the nitrogen oxide gases in the exhaust gas react with hydrocarbons, hydrogen, and carbon monoxide in the gas. The gaseous effluent coming from this first stage is then passed through the second stage, where unburned hydrocarbons and carbon monoxide react with oxygen in the gas, the oxygen having been added to the effluent from the first stage prior to its introduction into the second stage. In the proposed two-stage catalytic converters, each stage contains a type of catalyst that may be different from the catalyst in the other stage. The two-stage converter requires that the internal combustion engine which is emitting the exhaust gases be operated at fuel-rich conditions. Fuel-rich conditions ensure the presence of sufficient carbon monoxide and hydrocarbons to react with the nitrogen oxide gases in the first stage of such two-stage catalytic converter.
The catalyst in the first stage of such two-stage catalytic converter is called the NO.sub.x catalyst. This catalyst promotes the reduction of nitrogen oxide. It is to be understood that several different and sometimes competing reactions can and do occur as the exhaust gases flow over the NO.sub.x catalyst. The most important of these reactions are: EQU NO.sub.x + CO .fwdarw. N.sub.2 + CO.sub.2 ( 1) EQU no.sub.x + H.sub.2 .fwdarw. NH.sub.3 + N.sub.2 + H.sub.2 O (2) EQU 2co + o.sub.2 .fwdarw. 2co.sub.2 ( 3) EQU 2h.sub.2 + o.sub.2 .fwdarw. 2h.sub.2 o (4)
the first two of the above reactions are the reactions that are responsible for eliminating NO.sub.x as an exhaust pollutant. The NO.sub.x catalyst favors such reactions rather than Reactions (3) and (4). Preferably, the NO.sub.x catalyst favors such reactions over a wide range of temperatures, e.g., about 700.degree.F to about 1,700.degree.F. Reaction (2) is an undesirable reaction because ammonia that has formed in the first stage of the converter will be converted to nitrogen oxide in the second stage. This will defeat, at least in part, the purpose of the converter. Consequently, a desirable NO.sub.x catalyst either will not promote Reaction (2) or will minimize the formation of ammonia in favor of nitrogen. However, such selective catalysts which are viable and useful for the emission control systems to be employed in automobiles, as well as in the treatment of exhaust gases from non-mobile sources, have not been developed yet.
There has now been developed a process for maximizing the conversion of nitrogen oxides in the treatment of exhaust gases, which process employes a three-catalyst system.